Appearing slightly stunned, he said: "I'm sorry for that Mr Blair. I would like to find out how this gentleman managed to access the court through what is supposed to be a secure corridor.

"I will have an investigation undertaken about that immediately. I apologise."

Mr Blair added: "Can I just say on the record what he said about Iraq and JP Morgan is completely and totally untrue. I have never had any discussion with them about that."

The protester was escorted through the Royal Courts of Justice by security guards and was seen being driven away in a police van.

It is understood he managed to get past security-coded doors to access the judges' corridor leading to courtroom 73.

A Scotland Yard spokesman said: "He has been arrested on suspicion of breach of the peace. He's currently in custody at a central London police station."

A spokesman for Her Majesty's Courts and Tribunals Service said: "An investigation has been ordered into an incident at the Leveson Inquiry, Royal Courts of Justice. It would be inappropriate to pre-empt the findings of this investigation."

At the start of the afternoon session, Lord Leveson vowed to review security measures to guard against similar incidents.

"Considerable effort has been put into ensuring all witnesses can give their evidence in a safe and secure environment and I very much regret what has happened," he said.

"An investigation is being undertaken and I will be giving consideration to the steps that can be taken and should be taken against this particular intruder.

"Efforts will be redoubled to ensure that incidents of this nature don't recur. I repeat my apologies to Mr Blair and indeed to everyone else who was involved in or following our inquiry."